We have recently found that when we induced hypothyroidism in rats by initiating treatment with propylthiouracil (PTU) four days following a dose of DMBA, there was a significant decrease in the incidence of mammary carcinomas four months later, suggesting that hypothyroidism may impair the growth of DMBA-induced mammary cancer. We are currently investigating the effect of inducing hypothyroidism by administering PTU prior to the administration of the carcinogen, to examine the effect of hypothyroidism on the induction of mammary carcinoma by DMBA. We plan to examine hepatic binding of prolactin in rats bearing DMBA-induced mammary carcinoma, to investigate the possibility that these carcinomas may produce a substance which decreases the number of hepatic prolactin receptors. We plan to examine the capacity of DMBA-induced mammary carcinoma to convert thyroxine to triiodothyronine, since the production of the latter hormone, which is known to bind to receptors associated with nuclear DNA, could affect the rate of tumor growth.